1. Field of Invention
This invention pertains to spectrophotometers in general and particularly to high-resolution spectrophotometers.
2. Related Art
Spectroscopy is a widely used technique for analyzing various substances based on the fact that different substances show different absorption and emission bands.
FIG. 1 is an energy-level diagram for the hydrogen atom, showing origin of spectral lines for the Lyman, Balmer, and Paschen series. The quantum number n that labels the orbit radii also labels the energy levels. The lowest energy level, or the lowest “state,” is often referred to as the ground state (n=1). The higher states (n>1) are referred to as excited states. When an electron in an excited state jumps down to a lower state, it may give off energy in the form of radiation (e.g., a photon). Since the energy levels are quantized, the states that are involved in the jump can be determined based on the wavelength of the radiation that is emitted. For example, an electron jumping from n=3 to n=2 in the hydrogen atom give rise to a 656-nm line in the Balmer series, while an electron jumping from n=4 to n=2 would give rise to the 486-nm line. The emission is termed fluorescence and the transition between to states is said to be spin allowed if the states have the same spin multiplicity (i.e., both are singlets or both are triplets). If the spin multiplicity changes in the transition, the emission is termed phosphorescence.
Spectroscopy usually entails exciting a sample, for example by passing radiation through it, and determining the wavelengths that are released by the sample. The released emission band includes a pattern of varying intensities at different wavelengths, indicating the wavelengths where emission occurred.
Emission occurs following an absorption event if the upper state is not relaxed by a nonradiative collisional process (called “quenching”). In some cases, absorption, rather than emission, spectra is used in spectroscopy. To use absorption spectroscopy, radiation of known wavelength is passed through a sample. The radiation excites at least some of the electrons in the orbitals of the sample, which absorb energy to rise to a higher energy state. Due to some of the radiation being absorbed, the energy level of the radiation is lower at certain wavelengths after passing through the sample. The intensity pattern across the predetermined wavelength range is referred to as an absorption band, which shows the wavelengths at which absorption occurred.
The absorption band of a substance generally consists of several absorption bands arising from different vibrational motions of the molecule. Within each vibrational energy level are rotational energy levels. Since rotational energy levels are spaced closer together than the vibrational energy levels, a higher resolution is needed to observe the rotational energy levels.
Currently, the emission/absorption bands are determined by using a spectrophotometer that includes a grating and one or more detectors. Sometimes, a prism is used instead of the grating. The grating/prism separates the radiation from the sample into multiple rays based on wavelength, and each of the wavelength-specific rays is directed to a radiation sensor/detector. Various mirrors and other optical components are used to properly direct each radiation beam to the sensor/detector.
One of the disadvantages of the grating-based spectroscopy is that it is inconvenient to use when a high resolution is desired. To split the radiation from the sample into high-resolution wavelengths, multiple layers of grating/prism may be necessary. The increase in the number of gratings undesirably leads to a bulkier equipment and creates greater chances of inaccuracy. A compact spectrophotometer that can achieve a high resolution is desired.